In the modern mobile communication system, especially a mobile phone, to prolong the service life of a lithium battery, extend the talk time, and reduce the current consumption of a power amplifier, improving efficiency of the power amplifier is an effective way. Accordingly, there have been invented a plurality of methods for improving efficiency of the power amplifier, like class-E and class-F power amplifiers. Such amplifiers achieve very high efficiency when output power reaches a saturation value.
However, in the communication system, sometimes the system does not need high transmit power. To enable the system to have high efficiency in cases of power back-off, a multi-mode power amplifier is developed. The transmit power is set according to the power required by the communication system in the multi-mode power amplifier, such that the communication system still maintains high efficiency in cases of power back-off. Generally the multi-mode power amplifier supports three working modes, a low power mode (LPM), a medium power mode (MPM), and a high power mode (HPM). The multi-mode power amplifier is widely applied in 3G/4G mobile phones.
FIG. 1 shows a typical multi-mode power amplifier in the prior art. As illustrated in FIG. 1, the multi-mode power amplifier has three signal paths, i.e., a low power amplifier path, a medium power amplifier path, and a high power amplifier path. Each power amplifier path comprises a first switching circuit, an input matching circuit, a power amplifier module (QL, QM, or QH), an output matching circuit, and a second switching circuit. In the low power mode, the switching circuits are controlled to conduct the low power amplifier path, and cut off other power amplifier paths, and an input radio frequency signal RFIN is transmitted over the low power amplifier path and then a power-amplified radio frequency signal RFOUT is obtained. In the medium power mode, the switching circuits are controlled to conduct the medium power amplifier path, and cut off other power amplifier paths, and the input radio frequency signal RFIN is transmitted over the medium power amplifier path and then the power-amplified radio frequency signal RFOUT is obtained. In the high power mode, the switching circuits are controlled to conduct the high power amplifier path, and cut off other power amplifier paths, and the input radio frequency signal RFIN is transmitted over the high power amplifier path and then the power-amplified radio frequency signal RFOUT is obtained.
Such multi-mode power amplifiers can effectively improve efficiency of the communication system, however it has one or more of following disadvantages: 1. a separate power amplifier path needs to be developed with respect to each mode, thus the design period is long and the development cost is high; 2. the multi-mode power amplifier needs three power amplifier units having different powers, thus a large area is occupied; 3. design and debugging of the output matching circuit are complicated, thus the research and development cost is high; 4. the stages of the power amplifier are not changed in different power modes, such that the power gain in the three power modes has no notable difference, and a high requirement is imposed on a dynamic range of the transceiver.
Therefore, an improved multi-mode power amplifier is desired to overcome the above problem.